Geology and Genesis of Gold Mineralization In Kushmagane Area, Assosa Woreda, Western Ethiopia
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2016-06
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Geology of Western Ethiopian Shield is known by volcano-sedimentary terranes, gneissic terranes and ophiolitic rocks intruded by different granitoid bodies. This thesis study generally considered the geology and genesis of gold mineralization in Kushmagane area, a part of Western Ethiopian Shield and involved field investigations and petrographic and whole rock geochemical studies to determine genesis of gold and related processes. Kushmagane is covered by low grade metabasites and metagranitoids metamorphosed at greenschist facies. They consist of quartz veins which are both primary and metamorphic in their origin, the later lacking specific orientation. The metagranitoids are intruded by thin mafic intrusions that are oriented parallel to both regional foliation and the local contacts. They have high silica content (70.40–78.30wt %) and show increase in K2O and Na2O and decrease in Al2O3, MgO, MnO and Fe2O with increasing SiO2 which reflects the crystal fractionation in their evolution. Their REE pattern shows only slight variation between the abundances of LIL and HFS elements which indicates moderate percentage melting of the original material. The low content of Fe and Mg oxides in metagranitoids with their peraluminuous nature indicate crustal contribution. Mafic mineralogy and associated basic volcanic rocks on the other hand, infer mantle source. Therefore, the source for Kushmagane granitoids is likely both basaltic mantle derived parental magma and partial melting of continental crust. Kushmagane metabasites have high concentrations of MgO, Fe2O3 and CaO and low concentration of Na2O and K2O. Their REE abundance pattern shows nearly parallel trend in a negative general slope. Negative slopes combined with their tholeiitic nature infers that the Kushmagane basalts and andesites originated in the lower enriched mantle reservoir. The flat pattern in metabasalt suggests that the REE abundances in these rocks resulted from phases involving shallow fractionation (e.g., olivine), which do not fractionate the REE. Their tholeiitic nature and abundance of minerals like augite, pigeonite ad hypersthene with minor olivine indicates that the Kushmagane metavolcanics formed as oceanic slab subducted, reached sufficiently hot region and melted to leak into the crust. Gold mineralization in the area, is decorated by presence of sulphides like pyrite, pyrrhotite, chalcopyrite and covellite. Based on observed volcanic arc tectonic setting, ore petrography and alteration types, the mineralization is believed to be orogenic type hosted by metagranitoids and metabasites. The lower Au value in metabasites in the area may reflect that the rocks were also sources for metal and leached by hydrothermal fluids during metamorphism. The probable sources of fluids are proposed to be regional metamorphism of the mafic volcanic rocks, post to syn-tectonic intrusions and/or subducted oceanic crust.
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Kushmagane Area